The present invention relates to automatically controlled positioning devices in general and in particular to means for automatically controlling a reversible, motorized device, such as an antenna rotor, in response to an electronic signal from an external sensing device, such as a potentiometer coupled to the rotating shaft of a windvane.
In certain outdoor, scientific, engineering, or meteorological field tests or experiments it is necessary to use air sampling devices or other instruments which must be kept facing into the ambient wind during the sampling interval. Such instruments include, for example, some types of aerosol particle counters and visibility measuring instruments. When the ambient wind changes direction a means must be provided for rotating the instruments into the new wind direction. The problem becomes especially acute if it is desired to operate wind sensitive instruments on a long term, unattended basis, or if the wind is unusually variable in direction. In these cases it is convenient, and perhaps even necessary, to employ a means for automatically rotating the instruments into the changing wind in order to collect a sufficient quantity of valid data.
Existing means for automatically rotating various devices into the wind are limited to methods whereby the rotatable device is directly coupled to a wind driven vane. Such means are, of course, commonly used to keep windmills and propeller type anemometers facing into the wind. However, for heavy instrument packages as envisioned here, these means have at least the following distinct disadvantages:
First, the required vane size increases as the size or weight of the instrument package and platform increases. It is not uncommon for a hundred pounds or more of equipment to be installed on a rotatable platform of the type envisioned. Power and data cables leading to the instruments usually provide some resistance to rotation as well as some restoring torque which tends to rotate the system back as the cables get wound up around the platform or support pole. Thus, with a heavy instrument load or unfavorable power/data cable constraints, an impractically large vane may be required, especially if alignment is necessary in relatively low wind conditions. PA1 Second, with a vane controlled platform the aforementioned cables can easily become wrapped around the support pole or other nearby structures, especially in highly variable wind conditions. Needless to say, damage to the cables or instruments may result. If some kind of stops are provided to keep the platform from rotating more than 360.degree., for example, then the usefulness of the rotating platform is limited. In some situations the stops will prevent the platform from rotating the extra amount it needs to become properly aligned. This results in lost data or else it requires an operator to be in attendance to correct the situation. PA1 Third, a large vane would also be impractical in cases where there are unfavorable space constraints due, for example, to other nearby objects. PA1 First, unless the device prevented rotation by more than 360.degree. or so, there would be the previously mentioned danger of winding up the instrument cables around the support pole or other nearby objects. Even then, the stops may prevent the device from achieving proper alignment with the wind. PA1 Second, in calm, low, or intermittent winds the device may continually "hunt" for a non-existent wind. PA1 Third, in erratic or turbulent winds the device may "chatter" or try to respond to rapid fluctuations in wind direction about some mean value.
Devices such as servomechanisms or other means which have been used successfully in automatic steering applications could be adapted to the present application. However, without additional modifications or improvements these means have at least the following disadvantages: